A thermal processing chamber refers to a device that uses energy, such as radiative energy, to heat objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. For monitoring the temperature of the semiconductor wafer during heat treatment, thermal processing chambers also typically include radiation sensing devices, such as pyrometers, that sense the radiation being emitted by the semiconductor wafer at a selected wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
One major problem in the design of rapid thermal processing chambers having an optical temperature measurement system, however, has been the ability to prevent unwanted light radiated by the heater lamps from being detected by the pyrometric instrumentation. Should unwanted light not being emitted by the semiconductor wafer be detected by the pyrometer, the calculated temperature of the wafer may unreasonably deviate from the actual or true temperature of the wafer.
In the past, various methods have been used to prevent unwanted thermal radiation from being detected by the pyrometer. For instance, physical barriers have been used before to isolate and prevent unwanted light being emitted by the heater lamps from coming into contact with the pyrometer. Physical barriers have been especially used in rapid thermal processing chambers in which the heater lamps are positioned on one side of the semiconductor wafer and the pyrometer is positioned on the opposite side of the wafer.
Physical barriers, however, can restrict the system design. For instance, the physical barrier can restrict how the wafer is supported. In one embodiment, a light-tight enclosure is created below the wafer using a large diameter continuous support ring to hold the wafer at it edges. When a support ring is present, there can be overlap between the support ring and the edges of the wafer, which can lead to temperature non-uniformities in the wafer during heating cycles. Another problem can arise if the support ring or the wafer is warped even slightly. When this occurs, light can stray through the gap into the supposedly light-tight region. The stray light can induce errors in the pyrometer readings.
Besides physical barriers, spectral filters have also been used to limit the amount of light interference detected by the pyrometers. For instance, spectral filters can operate by removing light being emitted by the heater lamps at the wavelength at which the pyrometer operates. Preferably, spectral filters absorb unwanted thermal radiation while at the same time being transparent to the thermal radiation being emitted by the heater lamps that is necessary to heat the semiconductor wafer.
One type of spectral filter that has been used in the past is a window made from fused silica, such as silica doped with hydroxy (OH) ions. Fused silica glass is transparent to most light energy but is known to have several strong absorbing regions that are maximized at wavelengths of about 2.7 microns, 4.5 microns and at wavelengths equal to and greater than 5 microns.
Because certain OH-doped silica glass can effectively absorb light at wavelengths of 2.7, 4.5 and greater than 5 microns and is substantially transparent at many other smaller wavelengths of light, silica glass makes an effective spectral filter when the pyrometer contained within the thermal processing chamber is configured to sense thermal radiation at one of the above wavelengths.
Silica glass, however, is unfortunately not well suited to being used as a spectral filter in temperature measurement systems that contain pyrometers that sense thermal radiation at shorter wavelengths, such as less than about one micron. Specifically, in some applications, it is more advantageous and beneficial to operate pyrometers at relatively short wavelengths. In particular, by using pyrometers that operate at shorter wavelengths, the effects of wafer emissivity variations can be minimized providing for more accurate temperature determinations. Specifically, at lower wavelengths, silicon wafers are more opaque and the emissivity of the wafer is not significantly temperature dependent. The emissivity of the wafer is one variable that must be known with some accuracy in determining the temperature of wafers using pyrometers.
In addition to more precisely determining the temperature of wafers, pyrometers that operate at relatively shorter wavelengths are also generally less expensive and less complicated then pyrometers that are configured to operate at higher wavelengths. Further, pyrometers that sense thermal radiation at lower wavelengths generally operate very efficiently and can generate low noise measurements.
In the past, however, pyrometers that operate at lower wavelengths have been selectively used in thermal processing chambers due to the significant amount of stray light that can be detected in thermal processing chambers at lower wavelengths. As such, a need currently exists for a spectral filter that can efficiently absorb light energy at lower wavelengths, such as wavelengths less than about 2 microns.
The present invention is generally directed to an apparatus and method for heat treating semiconductor devices. The apparatus includes a thermal processing chamber adapted to contain a semiconductor wafer. A radiant energy source including at least one lamp is used to emit light energy into the chamber. At least one radiation sensing device is located within the thermal processing chamber and is configured to sense thermal radiation at a preselected wavelength being emitted by a semiconductor wafer being heat treated.
In accordance with the present invention, the apparatus further includes a spectral filter that is configured to absorb thermal radiation being emitted by the light source at the preselected wavelength at which the radiation sensing device operates. The spectral filter comprises a light absorbing agent. The light absorbing agent can be, for instance, a rare earth element, a light absorbing dye, a metal, or a semiconductor material. For example, in one embodiment, the spectral filter comprises a host material doped with a rare earth element. The rare earth element can be ytterbium, neodymium, thulium, erbium, holmium, dysprosium, terbium, gadolinium, europium, samarium, praseodymium, or mixtures thereof.
In an alternative embodiment, the spectral filter comprises a host material doped with a metal, such as a transition metal. Particular metals that can be used, include, for instance, iron and copper.
The host material can be a liquid, a glass, a crystal, a plastic or a ceramic. Of particular advantage, when the spectral filter contains a rare earth element, the spectral filter can be configured to absorb light energy at a wavelength of less than about 2 microns, such as from about 0.5 microns to about 1.5 microns, and particularly from about 0.6 microns to about 1.1 microns. For example, in one embodiment, the spectral filter can be ytterbium contained in a glass material in an amount of at least 0.5% by weight, and particularly in an amount of at least about 20% by weight. In this embodiment, the spectral filter can be configured to absorb light at a wavelength of between about 900 nm to about 1010 nm.
As described above, the amount of the light absorbing agent present within the host material can be measured in units of percentage by weight. For example, for many applications, the light absorbing agent can be present in the host material in an amount from about 0.5% to about 50% by weight. In some applications, however, it may be more appropriate to use atomic composition as a measure of concentration instead of weight percentages. For example, the light absorbing agent can be present in the host material at an atomic composition concentration (mole percent) of from about 0.5% to about 50%. The atomic composition concentration can vary depending upon the particular host material and the particular light absorbing agents selected.
In an alternative embodiment, the rare earth element can be in the form of a rare earth element compound, such as an oxide. The rare earth element compound can be contained in a ceramic material and used as a spectral filter in accordance with the present invention.
As described above, in another embodiment, the light absorbing agent can be a light absorbing dye. The dye can be, for instance, an organic salt dye, a nickel complex dye, a precious metal dye such as a platinum complex dye or a palladium dye, a phalocyanine dye, or an anthraquinone or a mixture thereof. Such dyes are also well-suited to absorbing light at wavelengths less than about 2 microns.
In addition to rare earth elements and light absorbing dyes, the spectral filter can also be made from a semiconductor material. The semiconductor material can be, for instance, gallium arsenide, aluminum arsenide, germanium, silicon, indium phosphide, or alloys of these materials, such as Si/Ge; AlAs/GaAs/InP.
Spectral filters made in accordance with the present invention can have an attenuation factor of at least 5 at the wavelength of interest. For instance, the spectral filter can have an attenuation factor of at least 103, and particularly can have an attenuation factor of at least 105 at the wavelength of interest. Further, the above attenuation factors can be obtained having a relatively thin material. For instance, the spectral filter can have a thickness of less than about 1 inch, and particularly less than about 100 mm.
The spectral filter can be positioned in association with the light sources in the apparatus of the present invention at various locations. For instance, in one embodiment, the spectral filter can be positioned in between the thermal processing chamber and the light sources. In an alternative embodiment, however, the spectral filter can be used to surround a lamp or a radiant energy filament. In still another alternative embodiment, the spectral filter can be incorporated into a reflector that is positioned behind the light sources.
Other features and aspects of the present invention are discussed in greater detail below.